Coaxial cables are well known and widely used as transmission lines for electrical signals in the video to microwave frequency range. Prior art coaxial cables may be of the rigid or flexible type. Rigid types may have a copper wire center conductor and a solid copper tubing outer conductor. The dielectric may be mostly gas in such arrangements, with only minimal insulating support structure holding the center conductor coaxial within the outer conductor.
A more familiar type of known coaxial cable is at least partially flexible and consists of a metallic solid or stranded wire center conductor surrounded by a solid, but usually not rigid, dielectric material having an outer conductor formed of a flexible, braided wire or metallic mesh layer held in coaxial relationship with the center conductor by the dielectric material.
Aircraft and space vehicles employ many electronic systems which, in turn, require signal interconnections. The signals may be pulses in the video frequency domain or radio frequency and microwave signals relating to the various communication and instrumentation functions required. Microwave signal conveyance is of primary importance.
It has always been important to minimize the weight of any apparatus carried by airborne vehicles and in fact is critical in space vehicles. The structural members of the vehicles themselves can be constructed of composites which provide the required strength but are lighter overall than the traditional materials of aircraft construction. The incentive for reduced vehicle weight is obvious in terms of overall mission performance, reduced operating costs and increased "payload" capability.
The technology associated with coaxial cables has not advanced apace with other advances in the aircraft/spacecraft technology. Such high density metals as copper, stainless steel and silver have continued to be used in coaxial cable fabrication. The common standard for microwave signal conveyance (RG-402) consists of a silver and copper clad stainless steel center conductor, a coaxial dielectric layer of polyethylene or polytetrafluoroethylene commonly known as "Teflon" (a Dupont trademark), and a solid copper tube outer conductor. That construction provides a rigid transmission line, formable to fit irregular spaces. The flexibility of coaxial cables of the shield braid outer conductor type is often not required and may even be detrimental in aircraft and space vehicles subject to vibration in their operational environments.
In the aforementioned solid, copper tube, outer coaxial conductor prior art configuration, the weight of the outer conductor is over half of the total weight of the cable.
It may be said to have been the general object of this invention to provide a coaxial cable structure of reduced weight, but with electrical performance comparable with prior art coaxial cables.
The so-called composite materials employing carbon (graphite) fibers have been employed as structural members where high strength-to-weight ratios are required. The electrically conductive properties of such fibers have also received prior art attention in various applications.
U.S. Pat. No. 4,687,882 discloses the loading of insulation material with conductive carbon fibers in a surge attenuating electrical cable.
U.S. Pat. No. 4,518,632 describes an undersea cable in which an inner conductor is formed of conductive fibers in a composite-like structure having good conductivity and tensile strength. Intercalation of graphite fibers is also indicated, this process enhancing conductivity.
The manner in which the invention employs the characteristics of carbon (graphite) fiber composites in a coaxial cable to reduce weight while providing comparable electrical performance vis-a-vis the prior art for such cables will be understood as this specification proceeds.